The present disclosure relates to filter circuits, and more particularly, to current-mode filter circuits including field effect transistors, and optical disk devices including the current-mode filter circuit in a signal processing path.
A filter circuit is a functional block essential for various signal processing systems. In particular, an analog filter circuit has an important role in shaping a signal waveform before analog-to-digital conversion and removing high-frequency noise for prevention of aliasing in an analog-digital hybrid LSI. In particular, in a signal band between several tens of megahertz and several hundreds of megahertz, a Gm-C filter including a transconductance circuit (hereinafter referred to as a Gm circuit) and a capacitive element is typically employed.
However, the Gm-C filter has the following problems (see “CMOS Continuous-Time Current-Mode Filters for High-Frequency Applications,” IEEE J. Solid-State Circuits, vol. 28, pp. 323-329, March 1993 (hereinafter referred to as NONPATENT DOCUMENT 1), and “A Current Mirror with Controllable Second-Order Low-Pass Function,” TECHNICAL REPORT OF IEICE ICD, Vol. 99, No. 316, pp. 71-77, 1999 (hereinafter referred to as NONPATENT DOCUMENT 2).
1. The parasitic pole of the Gm circuit in the filter is in the proximity of the pole of the filter. Therefore, it is difficult to achieve accurate frequency characteristics, particularly in a high frequency region.
2. It is difficult to ensure a wide dynamic range and linearity of a Gm-C filter operating in a voltage mode in a digital CMOS process with low voltage operation which is provided by recent microfabrication technology.
In an effort to address these problems, NONPATENT DOCUMENTS 1 and 2 have proposed filter circuits which operate in the current mode. In NONPATENT DOCUMENT 1, as shown in FIG. 2(a) in the document, an output current is fed back. In NONPATENT DOCUMENT 2, as shown in FIG. 8 in the document, a capacitive element is added to a gate grounded mirror circuit. The filter circuit of NONPATENT DOCUMENT 2 includes a smaller number of elements than that of NONPATENT DOCUMENT 1. Here, the filter circuit of NONPATENT DOCUMENT 2 having a configuration more similar to that of the present disclosure will be described in detail. FIG. 15 shows a configuration of the current-mode filter of NONPATENT DOCUMENT 2. In the current-mode filter of FIG. 15, N-channel transistors M200 and M203 form a current mirror pair. Each of the N-channel transistors M200 and M203 is driven by a bias current Ib0 from a current source. A P-channel transistor M201 functions as a gate grounded circuit whose gate is fixed to a constant voltage Vb0 and is driven by a bias current Ic0. The source of the P-channel transistor M201 is connected to the drain of the N-channel transistor M200, and the drain of the P-channel transistor M201 is connected to the gate of the N-channel transistor M200. As a result, the N-channel transistor M200 and the P-channel transistor M201 form a negative feedback loop. Capacitive elements Ci and Cg are connected to the drain and gate of the N-channel transistor M200, respectively. In this case, if the N-channel transistors M200 and M203 and the P-channel transistor M201 operate in their saturated regions, the transconductances (hereinafter referred to as gm) of the N-channel transistors M200 and M203 and the P-channel transistor M201 can be approximated by:gmn=√{square root over (2·βn·Ib0)}  (1)gmp=√{square root over (2·βp·Ic0)}  (2)where gmn is the gm of the N-channel transistor M200, M203, gmp is the gm of the P-channel transistor M201, βn is the transconductance parameter of the N-channel transistor M200, M203, and βp is the transconductance parameter of the P-channel transistor M201.
Here, if the drain of the N-channel transistor M200 is used as a current input (Ii) terminal, and the drain of the N-channel transistor M203 is used as a current output (lo) terminal, the input/output transfer function is represented by:
                                          Io            Ii                    =                      -                                          ω                ⁢                                                                  ⁢                                  0                  2                                                                              s                  2                                +                                                                            ω                      ⁢                                                                                          ⁢                      0                                        Q                                    ·                  s                                +                                  ω                  ⁢                                                                          ⁢                                      0                    2                                                                                      ⁢                                  ⁢                              ω            ⁢                                                  ⁢            0                    =                                                    gmn                ·                gmp                                            Ci                ·                Cg                                                    ⁢                                  ⁢                  Q          =                                                    gmn                ·                Ci                                            gmp                ·                Cg                                                                        (        3        )            
Expression 3 shows the transfer function of a second-order low-pass filter (hereinafter abbreviated to “LPF”), i.e., that the circuit configuration of FIG. 15 functions as a current-mode second-order LPF. As can also be seen from Expression 3, the value ω0 and the Q factor indicating the frequency characteristics are determined by gmp, gmn, Ci, and Cg.